Are thumb monkeys real

Yes...when you say that you mean Marmoset and Tamarin monkeys. I breed Cotton Top Tamarins..They are only the size of your finger when they are babies though...fully grown they both weigh about a pound, so they are still small.

They are a lot of work though, as they are highly intelligent creatures.

Please see the related link below.

Actually there is a type of monkey that is the size of your finger when full grown; it is the Pygmy Marmoset. There is a picture of two of them at the second related link below.